1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor and, in particular, relates to a multilayer ceramic capacitor that is excellent in reliability wherein the capacitance-temperature characteristic, i.e. a time-dependent change in capacitance, satisfies the EIA standard X8R characteristic (ΔC/C=±15% at −55 to +150° C.) and the resistance change ratio in an acceleration test is small (average life is long).
2. Description of the Related Art
Multilayer ceramic capacitors (multilayer chip capacitors), for example, are widely used as small-size, large-capacity, highly-reliable electronic components. In recent years, following miniaturization and higher performance of devices, the demand for further miniaturization, larger capacitance, lower price, and higher reliability has been increasing more and more with respect to the multilayer ceramic capacitors.
Normally, the multilayer ceramic capacitor is manufactured by alternately stacking in layers an internal electrode layer forming paste and a dielectric layer forming paste by the use of a sheet method, a printing method, or the like and simultaneously burning internal electrode layers and dielectric layers in a stack structure.
As a conductive material of the internal electrode layers, use has generally been made of Pd or a Pd alloy. However, since Pd is expensive, a base metal such as Ni or a Ni alloy, which is relatively cheap, has started to be used in recent years.
When the base metal such as Ni or the Ni alloy is used as the conductive material of the internal electrode layers, the internal electrode layers are oxidized if burning is carried out in the atmosphere. Therefore, the simultaneous burning of the dielectric layers and the internal electrode layers needs to be carried out in a reducing atmosphere. However, when the burning is performed in the reducing atmosphere, the dielectric layers are reduced so that the resistivity decreases. In view of this, an unreducible dielectric material has been developed.
However, with respect to a multilayer ceramic capacitor using the unreducible dielectric material, there is a problem that the IR (insulation resistance) extremely decreases due to application of an electric field (i.e. the IR life is short) so that the reliability is lowered.
Further, there arises a problem that when the dielectric is exposed to a DC electric field, a relative permittivity εr decreases with the lapse of time. There is also a problem that there are those instances where a capacitor is used while being superimposed with a DC voltage and, in general, when a DC voltage is applied to a capacitor having a dielectric made of a ferroelectric material as a main component, there occurs degradation in characteristic where the permittivity changes depending on the applied DC voltage (which is called a DC bias characteristic) or in capacitance-temperature characteristic of the DC voltage application (which is called a Tc bias characteristic). Particularly, when dielectric layers are reduced in thickness for achieving miniaturization and increase in capacitance of a multilayer ceramic capacitor in response to the demand in recent years, since electric fields applied to the dielectric layers upon the application of DC voltage increase, the problem becomes remarkable wherein the time-dependent change in relative permittivity εr, i.e. the time-dependent change in capacitance, extremely increases or the Tc bias characteristic degrades.
Further, the multilayer ceramic capacitor is also required to be excellent in temperature characteristic and is particularly required, depending on a use, to have a flat temperature characteristic under severe conditions. In recent years, various electronic devices such as an engine electronic control unit (ECU), a crank angle sensor, and an antilock brake system (ABS) module that are mounted in an engine room of an automobile are required to be excellent in circuit temperature stability because those electronic devices are used for stably executing an engine control, a driving control, and a braking control, respectively.
It is expected that, in an environment where those electronic devices are used, the temperature drops to about −20° C. or below in the winter in a cold district while it rises to about +130° C. in the summer at the start of the engine. Recently, there is a tendency to shorten wire harnesses that connect between electronic devices and their control object devices and therefore there are those instances where the electronic devices are disposed outside a car. Accordingly, the environment for the electronic devices is becoming more severe. Therefore, capacitors for use in those electronic devices are each required to have a flat temperature characteristic over a wide temperature range.
As a temperature compensation capacitor material excellent in temperature characteristic, there are generally known (Sr, Ca)(Ti, Zr)O3 based compositions, Ca(Ti, Zr)O3 based compositions, and so on. However, since these compositions each have a very low relative permittivity, it is practically impossible to manufacture a capacitor having a large capacitance.
As a dielectric porcelain composition having a high permittivity and a flat capacitance-temperature characteristic, there is known a composition containing BaTiO3 as a main component, Nb2O5—Co3O4, MgO—Y, a rare-earth element (Dy, Ho, or the like), Bi2O3—TiO2, and so on. However, although the BaTiO3 based high-permittivity material can satisfy the EIA standard X7R characteristic (ΔC/C=±15% at −55 to +125° C.), it cannot be said that it is good enough for use in the automobile electronic devices used in the foregoing severe environment. For the electronic devices used in the foregoing severe environment, there is required a dielectric porcelain composition that satisfies the EIA standard X8R characteristic (ΔC/C=±15% at −55 to +150° C.).
Under these circumstances, the present assignee has already proposed dielectric porcelain compositions each having a high relative permittivity and the X8R characteristic and capable of being burned in a reducing atmosphere (Japanese Patent No. 3348081, Japanese Patent No. 3341003, JP-A-2001-31467).
However, following further miniaturization of the multilayer ceramic capacitors, the demand for higher reliability is becoming more intense and further improvement in performance is required.
The present invention has been made under these circumstances and has an object to provide a multilayer ceramic capacitor that is excellent in reliability wherein the capacitance-temperature characteristic satisfies the EIA standard X8R characteristic (ΔC/C=±15% at −55 to +150° C.) and the resistance change ratio in an acceleration test is small (IR average life is long).